1. Field of the Invention
The present invention relates to a nuclear medical apparatus capable of obtaining a three-dimensional distribution of a radioisotope in a subject by administrating the subject with a radioactive medicine labeled with the radioisotope and detecting the gamma rays given off from the radioisotope.
2. Description of the Related Art
There is known a single photon emission computed tomography (SPECT apparatus) as one of the nuclear medical apparatuses that, by administering a radioactive medicine labeled with a radioisotope to a subject, a distribution of the radioisotope in the subject is imaged from the distribution state of the gamma rays given off from the radioisotope. The SPECT apparatus is an apparatus that, by administering to a subject a radioisotope of 99mTc, 201Tl, 123I or the like, (termed a single photon nuclide), to radiate one gamma ray upon collapse, and counting the gamma rays given off upon collapse of the administered radioisotope, photon by photon, at a body exterior, thereby reconstructing a tomographic image of the subject from a concentration distribution of the nuclide on a tomographic plane.
Of among the radioactive medicines labeled with radioisotopes, there are nuclides that give off a plurality of energies. It is known that, where collecting gamma rays from such a nuclide giving off a plurality of energies, the quality of a tomographic image can be improved by setting up a collection window at around the energy at which the count value of the gamma rays given off from the radioisotope to be measured (termed a peak energy) maximizes and by gathering only the gamma rays fallen within the collection window. Meanwhile, there is known, say, a triple energy window (TEW) scheme as one of scattering-ray corrections for the effect of scattering gamma rays fallen within the collection window (see JP-A H11-38145, for example). In this scheme, besides the main collection window, sub-collection windows are provided on the higher and lower sides than the main window so that the effect of scattering rays, upon the count value of the gamma rays in the main window, can be corrected from the count values of the gamma rays as counted in the two sub-windows.
In the SPECT apparatus, count can be simultaneously made on the radioisotopes respectively contained in a plurality of radioactive medicines. In such a case, collection windows are set up at around the peak energies respectively corresponding to the plurality of radioisotopes so that the gamma rays given off from the radioisotopes can be separated by individually gathering the gamma rays fallen within the collection windows established.
In the meanwhile, the radiation detector (e.g. structured by an NaI scintillator and a photoelectron multiplier) used on the SPECT apparatus has a collection efficiency different depending upon the energy of the gamma rays given off from the radioisotope. Collection efficiency is unique to the structure of the radiation detector wherein collection efficiency has a change ratio differing relying upon the gamma ray energy due to the type of a collimator arranged on the radiation detector.
In this situation, in the existing medical apparatus, the difference of energy-based collection efficiency is not corrected thus measuring higher-energy and lower-energy gamma rays equally one in count. The SPECT apparatus already possesses the function of storage in different files for the collection energy windows. Although it is conventionally possible to merely take reciprocals of the images of within the files, there encounters a problem in displaying, on the same image, the gamma rays gathered in different collection energy windows.
For example, where administering a radioactive medicine to a body interior and quantitating the administered radioactive medicine (measuring an integration site and quantity of the radioactive medicine) thereby quantitating a protein (e.g. an organ or a tissue) using the radioactive medicine, it is essentially required to correct for the gamma-ray count value by taking account of the effect of collection efficiency different from energy to energy and from collimator to collimator. Likewise, where administering a plurality of radioactive medicines to a body interior and simultaneously quantitating the plurality of administered radioactive medicines (measuring integration sites and quantities of the radioactive medicines), it is significantly important to correctly discriminate the gamma-ray based on the collection window in which the gamma rays are gathered. In such a case, it is essentially required to correct for the gamma-ray count value by taking account of the effect of collection efficiency different from energy to energy and from collimator to collimator.
Furthermore, when correcting for scattering rays according to the TEW scheme, correction is based on the gamma-ray count values concerning the collection windows provided around the main window. However, in the current situation, the energy-based and collimator-based differences of collection efficiencies are not reflected upon the sub-windows on the higher and lower energy sides than the main window.